diff --git a/pkgs/racket-test-core/tests/racket/object.rktl b/pkgs/racket-test-core/tests/racket/object.rktl
index f3c54d11d0..8c75161bd6 100644
--- a/pkgs/racket-test-core/tests/racket/object.rktl
+++ b/pkgs/racket-test-core/tests/racket/object.rktl
@@ -1999,7 +1999,11 @@
       (define/public (n) 2)
       (super-new)))
   (test 3 'mixin-with-local-member-names (send (new (mix c%)) x)))
-  
+
+(err/rt-test (mixin (object%) () (super-new))
+             exn:fail:object?
+             #rx"not an interface")
+
 ;; ----------------------------------------
 ;; Class contracts & generics
 
diff --git a/racket/collects/racket/private/class-internal.rkt b/racket/collects/racket/private/class-internal.rkt
index 816f9ac59a..881fa42f6e 100644
--- a/racket/collects/racket/private/class-internal.rkt
+++ b/racket/collects/racket/private/class-internal.rkt
@@ -4697,7 +4697,9 @@ An example
                          [(or (as-write-list? val)
                               (as-value-list? val))
                           (apply string-append
-                                 (for/list ([v (in-list (as-write-list-content val))])
+                                 (for/list ([v (in-list (if (as-write-list? val)
+                                                            (as-write-list-content val)
+                                                            (as-value-list-content val)))])
                                    (format (if (as-write-list? val)
                                                "\n   ~s"
                                                "\n   ~e")